The present invention relates to thermodynamic conversion apparatus and, more particularly, to thermodynamic conversion apparatus employing air as a working medium.
Some thermodynamic conversion or chemical processes produce large quantities of hot byproduct gasses containing sufficient heat energy to make it economically attractive to collect and use the heat energy. For example, a gas turbine burns fuel with air in a combustor to produce hot gasses. The hot gasses are expanded in a turbine to produce torque. Part of the torque is used to drive an air compressor feeding compressed air to the combustor. The remainder of the torque is available on an output shaft.
Exhaust gasses exit the gas turbine at a temperature on the order of 1000 degrees F. When the hot exhaust gasses are vented to the atmosphere, the thermodynamic efficiency of such a gas turbine is quite low as compared to, for example, a steam turbine. Even with its relatively low efficiency, the rapid startup and flexible operation offered by gas turbines have encouraged its use as a prime mover for peaking power generation and in marine propulsion systems.
Heat recovery steam generators are commonly paired with gas turbines to form a steam and gas turbine combined cycle system. The heat recovery steam generator absorbs a substantial part of the heat energy in the exhaust gasses to produce steam which is available for use by any convenient using process. A combined cycle system of this sort has an overall efficiency which compares favorably with that of steam turbines.
A steam and gas turbine combined cycle system has a number of drawbacks. Steam is a difficult medium to contain and handle. For example, in order to avoid corrosion and scale in the water side of the heat recovery steam generator, the make-up water must be carefully treated to ensure its purity. Equipment for achieving such water purity is expensive. In addition, heat recovery steam generators are large devices requiring great capital investment and substantial real estate.
A heat recovery steam generator responds far more slowly to required changes in output than does the gas turbine. An extended period of, for example, two hours, is required for a cold start of the heat recovery steam generator during which output power is derived only from the gas turbine while the gas turbine exhaust heats the water in the heat recovery steam generator to operating temperature and pressure. A corresponding period of operation, without the benefit of output from the heat recovery steam generator, is required for shut-down. If the steam and gas turbine system is operated for 16 hours per day, during four of those hours, the gas turbine operates alone either to bring the heat recovery steam generator up to operating conditions, or to permit it to cool to its quiescent condition. Thus, although the start-up and operational flexibility offered by the gas turbine is retained, the improved efficiency and power output offered by a heat recovery steam generator is available for significantly less than its total operating time.
I have discovered that maximum thermodynamic efficiency is attained in absorbing heat from the hot gasses with a minimum thermal gradient between the medium giving up the heat and the medium receiving the heat. Steam generation necessarily requires a substantial regime where the medium on the water side of the heat exchange remains at a constant temperature while the water evaporates to steam. This process violates the above rule about minimum thermal gradient and thus degrades the amount of energy which can be absorbed from the hot gasses and delivered to a using process.
Numerous manufacturing and chemical processes require a plentiful supply of hot, compressed, unvitiated air. Unvitiated air is air whose oxygen has not been subjected to a combustion process wherein a substantial part of the oxygen is replaced by combustion products (usually including carbon dioxide, carbon monoxide, unburned fuel and, in the case of solid-fuel combustion, with fuel, ash and slag particulates.) A steam and gas turbine combined cycle system does not make such a supply of unvitiated air available without requiring separate hardware to produce it.
Stack gasses leave a steam and gas turbine combined cycle system at about 270 degree F. Although this represents a substantial energy waste, further heat recovery from the stack gasses conventionally is impractical due to the presence of corrosive compounds, principally sulfur compounds, which precipitate out of the gasses if their temperatures are reduced too far. An exhaust flow of heated air, on the contrary, requires no such temperature constraint. The temperature of heated air can be reduced as low as desired without suffering the consequences of precipitation of harmful compounds. At most, reducing the temperature of heated air permits the precipitation of water contained therein. Such precipitated water may be a valuable economic commodity in some environments.